feat: Add validating addmission webhook for ALB Ingress annotations

[kamilprzybyl]

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What this PR does / why we need it:

This PR introduces a Validating Admission Webhook to enforce mandatory STACKIT annotations (like alb.stackit.cloud/network-mode) on Ingress resources. It prevents invalid Ingresses from being created in the cluster if they do not meet the ALB controller's requirements.

Which issue(s) this PR fixes:
Fixes #

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Breaking changes:

[ske-prow]

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[ske-prow]

@kamilprzybyl: The following test failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
pull-cloud-provider-stackit-verify	02fdbf8	link	true	/test pull-cloud-provider-stackit-verify

Full PR test history. Your PR dashboard. Command help for this repository.
Please help us cut down on flakes by linking this test failure to an open flake report or filing a new flake report if you can't find an existing one. Also see the gardener testing guideline for how to avoid and hunt flakes.

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